This invention relates generally to sewing machines and directly concerns a new and improved drive control system for sewing machines wherein an electric motor drive can be controlled at selected speeds and, if required, positioned with reasonable accuracy.
Most common industrial sewing machines in use today were originally equipped with alternating current induction type motors, many of which include integral clutch and brake mechanisms, as well as an inertia-storing flywheel for obtaining faster acceleration and deceleration. During engagement of the motor with the sewing machine drive shaft through the clutch, the inertia of the flywheel supplies energy to bring the machine mechanism toward operating speed. For controlling deceleration, a friction type of brake is often used. Other types of drives may include drive belts and pulleys for coupling the motor to the sewing machine.
Although induction motor drives are still conventional and in wide use, the various mechanical means used to improve their start and stop characteristics are neither entirely effective nor free of frequent mechanical failure. All components, for example, must be able to withstand the impact loads arising from rapid engagement of the clutch. Furthermore, loud noises resulting from operation of the clutch and brake are disturbing to the operators.
Recent developments have been made which avoid many of those shortcomings. To that end, the induction motor and its associated mechanisms were abandoned in favor of low-inertia, direct current motors of the type disclosed in U.S. Pat. No. 2,920,238. These motors have a surface wound rotor, usually comprised of printed circuit windings, which has very little inertia of its own and yet is capable of yielding substantial driving torque. It is thus a characteristic of such motors that they accelerate and decelerate rapidly, even under load.
In the drive systems now known to implement low-inertia direct current drive motors, however, unnecessarily complex and expensive control circuits have provided new complications of their own. Although these drive systems perform well when all components are operating satisfactorily, their complexity has multiplied not only the initial investment in each system, but also the day-to-day maintenance costs. One specific drawback of this complexity is the necessity for highly trained service personnel in case of technical faults and, thus, sewing machine users are not usually able to restore the drive system to working order within a short time after breakdown.
Another disadvantage of the known systems in their requirement for multiple phase power. Many users of sewing machines demand compatability with three-phase power, and those drive systems now in use which have the low inertia dc motors have met that requirement by drawing all power from a three-phase transformer. Also, each phase, after being rectified, drives an SCR which serves as the controllable element between the motor and the power transformer. Various motor speeds are obtained by regulating the conduction duty cycle of the SCR. Two major deficiencies inhere such SCR motor control. First, the switching of the SCR's at times other than the zero crossing point for the ac signal causes large transient spikes to be applied to the motor. Such transients could damage the motor or overload circuit fuses. Second, the discontinuous nature of the drive signal waveform means that large harmonic components are present, which cause joule heating without contributing any useful work.
Circuits in the prior art drive systems for regulating needle positioning have tended to be unusually intricate, incorporating many binary logic elements such as flip-flops, gates, monostable multivibrators and pulse circuits for sensing drive speed. Thus, the needle positioning means has only compounded the complexity of the already complicated drive system. The cost of these systems, for example, may amount to two or three times the cost of a direct drive system according to the invention, and maintenance problems are aggravated.
Most speed control devices for sewing machine drives function by stepping the motor in discrete jumps from one speed to another. Furthermore, most variable speed drive systems have not been readily adaptable to existing equipment and have not provided the wide range of operating speed and control desired by sewing machine operators, and even the improved dc drives may use electromechanical braking to get acceptable stepping and positioning response. In some sewing machine drive control circuits, a rheostat is used to develop a variable control voltage; as is well known, such resistive devices present an appreciable continuous power train, even under no-load or low-load conditions.
A primary object of the invention is therefore to provide a direct drive system for sewing machines and similar equipment which overcomes the many disadvantages associated with systems now known.
Another important object of the invention is to provide a sewing machine drive control which is continuously variable in speed from full stop to maximum speed.
A further object is the provision of a drive control system for sewing machines which is more reliable and far simpler than those of the prior art.
An additional important object of the invention is to improve the needle positioning means for sewing machines and to render such means compatible with the drive control means of the invention.
An improved bobbin winding mechanism is also disclosed.